The invention relates to a directional antenna for high frequencies, capable of radiating decimetric or centrimetric waves in accordance with a narrow angular range whose median angle .theta..sub.o belongs substantially to the range 5.degree.-85.degree.. The antenna is formed by a solid of a substantially rectangular parallelepiped shape having a thickness e, a length L and a width l.sub.t, and comprises a substrate material having a thickness h and a dielectric constant .epsilon..sub.r, whose surface is almost entirely metal-plated. The antenna is intended to be flush-mounted on the exterior wall of a missile or an aircraft.
A specific field of application of such antennae is that of proximity fuses or missile radar equipment for transmission or reception. In this application a radiation diagram is desired in the form of a conical semi-nappe or a full conical nappe directed towards the front of the missile, with a pronounced ascending edge of the major lobe, which is inclined at an angle .theta..sub.o with respect to the axis of the missile. For this type of application the angle .theta..sub.o is in the range 20.degree.-70.degree. and the side lobes which have a radiation angle near 90.degree. must have a reduced amplitude of at least 15 to 20 dB with respect to the major lobe in such a manner that particularly the radiation towards the ground is attenuated to the highest possible extent. On the other hand, an aerial mounted on the outer wall of a missile must be small and must assume as much as possible the generally curved shape of this wall so as to avoid a negative effect on the aero-dynamic properties of this wall and to ensure a good mechanical bond and a limited degree of heating in consideration of the high speed at which the missile moves through the air. A main characteristic of this type of antennae is to exhibit a wide roll diagram, whose shape, when the antenna is mounted on a cylinder, is very near to a cardioid. Moreover, because of the travelling wave, the diagram in the meridian plane is directional and orientable as a function of the operating frequency. Finally, the matching band must be on the order of 15% for a standing-wave ratio of 2 (90% of the transmitted power).
Directional antennae of the type described in the opening paragraph are disclosed more specifically in the article "Travelling-wave slot antennae" by J. H. Hines et al. published in PROCEEDINGS OF THE IRE (pages 1624-1631), volume 41, no. 11, November 1953, IEEE NEW YORK (U.S.). This article relates particularly to a slotted directional antenna for very high frequencies, filled with a dielectric material. More accurately, this prior art antenna is constituted by a groove perpendicular to the ground plane, which causes the antenna to be too wide (deep) and too thick. The excessive width (depth) makes it difficult to flush-mount the antenna to the wall of an aircraft and the excessive thickness renders it impossible to control perfectly the radiation diagram of the antenna, even when grouped in an array (especially when the dielectric is formed by the air). As a matter of fact, an excessively large thickness gives rise to losses, more specifically losses due to very high radiations, so that comparatively short antennas are ultimately chosen, while a good directional antenna of this type must still be capable of radiation at a distance from the feed point equal to several times the length of the wave in the air. It should furthermore be noted that arranging the antenna perpendicularly to the ground plane makes it impossible to obtain a wide roll diagram, particularly when the antenna is formed by an array of at least two elementary antennas.
Alternatively, thin-structure antennas are known which make use of printed circuit technology, more specifically of the type described in French Patent Application No. 2,481,526, filed by Applicants and corresponding to U.S. Pat. No. 4,371,877. This antenna has the advantages that it is of reduced bulk and that it is easy to realize, as is desired, but it is not directional. It operates as a cavity in the resonant mode, and radiates like a dipole, its diagram having the shape of a semi-torus (angle .theta..sub.o is equal to 90.degree.), whose axis merges into the axis of the air-craft on which it has been mounted. Further known antennas such as "pavement" antennas or flat antennas, employing printed circuit techniques also operate as dipoles, preventing them from being used in the field of application desired for the present invention.